Sets this to be activatable.
If an item is activatable, double-clicking on the item, using the Return key or calling [method@Gtk.Widget.activate] will activate the
item. Activating instructs the containing view to handle activation. `GtkListView` for example will be emitting the [signal@Gtk.ListView:
GtkListItem:activate] signal.
By default, listitems are activatable.
| this |
a listitem |
| activatable |
if the item should be activatable |